Commercial and general aviation aircraft often utilize high lift devices called leading trailing edge devices, or slats and flaps, respectively. A slat forms the leading edge of an aircraft wing and may be controllably commanded to extend and retract during takeoff and landing to increase lift during low speed flight. A flap forms the trailing edge of the aircraft wing and may be controllably commanded to extend or retract during takeoff and landing to increase lift. Typically, the slat has two configurations which are “deployed” or “stowed.” Flaps, on the other hand, may have multiple configurations or “detents,” which means the flap extends to various detents with each extension progressively giving the wing a greater surface area. When the flap is fully retracted it is considered to be in the “stowed” position. FIGS. 1A-1E show a cross-sectional view of an aircraft wing 10 with a flap system 20 and a slat system 30, in which each system is shown in various configurations as described above.
The slat system 30 includes a slat 32, a track 34, guide rollers 36 and a penetration 38 in the front spar 40. For purposes of orientation and completeness, ribs 42 are shown with stringers 44 in place. During retraction, the track 34 extends into the penetration 38, which is sealed with a “slat” or “track” can (FIG. 2). One purpose of the slat can is to prevent fuel leakage from a wing fuel tank (not shown) through the front spar 40.
Fleet experience has found that slat cans can collect water due to condensation or other types of moisture ingress. Therefore, conventional slat cans may need to be periodically drained to function properly. In at least one instance, the slat track has been damaged during retraction due to ice buildup within the slat can combined with inadequate drainage of moisture from the slat can, and where the track eventually drove the built-up ice through an aft wall of the slat can. Conventional certification, safety and maintenance guidelines include a functional test to verify the slat system can withstand drain failure and ice buildup.
Conventional slat cans are typically made from either cast or welded aluminum. Cast slat cans are made using investment casting techniques that are well known in the industry. Welded slat cans utilize machined or pre-formed metallic details that are pieced together in a tooling jig and subsequently welded. It should be appreciated that welding and casting processes for primary aircraft structure (described below) require significant x-ray, penetrant, and other inspection methods to verify porosity levels and acceptability at the production level, as well as periodic inspections over an operational life of the slat system. It is well known that metallic casting and welding processes are susceptible to porosity, and that both processes may generate undesired scrap rates.
The difference between primary structure and secondary or non-structural components is that primary structure must be certified to sustain design ultimate flight and ground loads. Primary structural components contribute significantly to carrying flight, ground, and pressurization loads, and whose failure could result in catastrophic failure of the airplane. Components classified as primary structure must, at least in they United States, be designed with an ultimate safety factor and a level of damage tolerance that maintains a threshold amount residual strength, fail safe ability, and limited crack growth rates for the operational life of the airplane.
Components that are not classified as primary structure are either classified as secondary structure or non-structural, which means they are not considered essential to safe flight. Loss of these components could result in benign functional issues, such as a partial loss of air conditioning or a malfunctioning overhead bin latch, for example. It follows that the processes that govern the manufacture and maintenance of secondary and non-structural components are, by definition, less stringent and generally not acceptable for producing primary structural components.
By way of example, manufacturing processes for producing reinforced composite or plastic secondary or non-structural elements may include, but are not limited to, keyed metallic internal tooling, metallic external tooling with a bladder, resin transfer molding (RTM), plaster, salt or dissolvable foam mandrel layup, vacuum bag layup. However, each of these processes have drawbacks such as, but not limited to, ply wrinkling, undesired ply compaction during cure, ply location accuracy, resin rich regions.
With exception of RTM, the above processes either utilize internal tooling with a vacuum bag, or external tooling with no way to control ply location. In the case where internal tooling is used, the plies compact against a hard tool and wrinkles form. The size of the wrinkles can be modeled by calculating the difference in compaction from the uncured ply thickness to the “as cured” ply thickness.
In the case where an external tool is used with a bladder or silicone bag, internal plies need to be accurately located and laid inside the tool or mold, which makes these processes unfit for an airplane production environment where close tolerances and consistency is required.
For the RTM process, plies are laid on keyed metallic inserts and then inserted into a metallic mold. In this case, tolerance between the keyed metallic inserts has to be closely maintained to the point where it is unrealistic to expect repetitive tolerances in the airplane production environment. Once the tolerances are out of specification, either due to resin build up on the pieces, or tool wear, or, if they were improperly designed or fabricated in the first place, then excess resin pools in the openings, or plies are pinched in the narrowed sections. In the first case, resin micro cracking can lead to a failure. In the second case, the plies are weakened and not suitable for the requisite structural loads.